A feedback controlled thermal wall plate designed to investigate thermal boundary layer flows is described and validated. The unique capabilities of the design are the ability to modify the thermal boundary conditions in a variety of ways or to hold the wall-temperature fixed even when the flow above the wall is unsteady and strongly three-dimensional. These capabilities allow for the generation and study of thermal transport in nonequilibrium boundary layer flows driven by different perturbations and of varying complexity. The thermal wall plate and the experimental facility in which the thermal wall plate is installed are first described. The wall-plate is then validated in a zero-pressure-gradient (ZPG) boundary layer flow for conditions of a uniform wall temperature and a temperature step. It is then shown that the wall temperature can be held constant even when a hemisphere body is placed on the wall that produces large localized variations in the convective heat transfer coefficient. Last, since the thermal wall plate is intended to support the study of thermal transport in a variety of nonequilibrium boundary layer flow, several possible experimental configurations are presented and described.

For vibro-acoustic applications, a turbulent wall pressure (TWP) fluctuations model was derived. The model is based on the resolution of Poisson's equation. The pressure is characterized in time and space through its spectrum in the frequency wave-number domain. The developed model follows trends commonly observed using Corcos model in a large frequency range but also shows new behaviors for low and high frequencies. The radiated noise due to TWP fluctuations is then computed in accordance with the form of the TWP spectrum. A specific computational methodology is proposed to perform the calculation without introducing limiting hypothesis on the radiated impedance.

An efficient large-eddy simulation (LES) approach is investigated for laminar-to-turbulent transition in boundary layers. This approach incorporates the boundary-layer stability theory. Primary instability and subharmonic perturbations determined by the boundary-layer stability theory are assigned as forcing at the inlet of the LES computational domain. This LES approach reproduces the spatial development of instabilities in the boundary layer, as observed in wind tunnel experiments. Detailed linear growth and nonlinear interactions that lead to the H-type breakdown are well captured and compared well to previous direct numerical simulation (DNS). Requirements in the spatial resolution in the transition region are investigated with connections to the resolution in turbulent boundary layers. It is shown that the subgrid model used in this study is apparently dormant in the overall transitional region, allowing the right level of the growth of small-amplitude instabilities and their nonlinear interactions. The subgrid model becomes active near the end of the transition where the length scales of high-order instabilities become smaller in size compared to the given grid resolution. Current results demonstrate the benefit of the boundary-layer forcing method for the computational cost reduction.

In this work, we have performed the flume study to analyze the high-order velocity moments of turbulent boundary layer with and without downward seepage. Sediment transport experiments were done in the laboratory for no seepage (NS), 10% seepage (10%S), and 15% seepage (15%S) cases. Measures of streamwise velocity variance were found increasing with seepage, which lead to increase in sediment transport with seepage. Results show that the variance of streamwise velocity fluctuation follows logarithmic law with distance away from the bed, within inner layer. This observation is also valid for even-order moments obtained in this work. The results show that the (2p-order moments) 1/p also follows logarithmic law. The slopes A p in the turbulent boundary layer seem fairly unaffected to NS and seepage flow but follows nonuniversal behavior for NS and seepage runs. The computed slope based on the Gaussian statistics does not agree well with the slope obtained from the experimental data and computed slope are reliable with sub-Gaussian performance for NS flow and super-Gaussian behavior for seepage flow.

The maxima of Reynolds shear stress and turbulent burst mean period time are crucial points in the intermediate region (termed as mesolayer) for large Reynolds numbers. The three layers (inner, meso, and outer) in a turbulent boundary layer have been analyzed from open equations of turbulent motion, independent of any closure model like eddy viscosity or mixing length, etc. Little above (or below not considered here) the critical point, the matching of mesolayer predicts the log law velocity, peak of Reynolds shear stress domain, and turbulent burst time period. The instantaneous velocity vector after subtraction of mean velocity vector yields the velocity fluctuation vector, also governed by log law. The static pressure fluctuation p ′ also predicts log laws in the inner, outer, and mesolayer. The relationship between u ′ / U e with u/U e from structure of turbulent boundary layer is presented in inner, meso, and outer layers. The turbulent bursting time period has been shown to scale with the mesolayer time scale; and Taylor micro time scale; both have been shown to be equivalent in the mesolayer. The shape factor in a turbulent boundary layer shows linear behavior with nondimensional mesolayer length scale. It is shown that the Prandtl transposition (PT) theorem connects the velocity of normal coordinate y with s offset to y + a, then the turbulent velocity profile vector and pressure fluctuation log laws are altered; but skin friction log law, based on outer velocity U e , remains independent of a the offset of origin. But if skin friction log law is based on bulk average velocity U b , then skin friction log law depends on a, the offset of origin. These predictions are supported by experimental and direct numerical simulation (DNS) data.

Present investigation deals with the interaction of an incident oblique shock wave on a turbulent boundary layer over a wavy surface. The oblique shock wave was generated by an 8 deg wedge in a freestream Mach number of 2.0. Three-dimensional (3D) Reynolds-averaged Navier–Stokes (RANS) equations with k–ω shear stress transport (SST) turbulence model were used for numerical computation. The computed results are in good agreement with the experimental measurement and direct numerical simulation (DNS) data in case of the interaction of an oblique shock with plain flat plate. To identify the effect of surface waviness on shock wave/turbulent boundary layer interaction (SWBLI), a section of the flat plate was replaced by a wavy surface. Computations have been conducted for different magnitudes of wavy amplitude. Further, the wavelength of the wavy surface has been varied. Results showed that the presence of wavy surface induces supplementary shock and expansion waves in the flow field, which are referred as topographic waves. This supplementary system of waves interacts with the counterpart of intrinsic SWBLI in a complex manner. Flow structure, separation behavior, and aerodynamic characteristics are studied. It is revealed that the amplitude is dominant than the wavelength of waviness in case of SWBLI on a wavy surface.

In the present study, the optimal two-dimensional (2D) tripping technique for inducing a naturally fully developed turbulent boundary layer in wind tunnels has been investigated. Various tripping techniques were tested, including wires of different diameters and changes in roughness. Experimental measurements were taken on a flat plate in a wind tunnel at a number of locations along the flat plate and at a variety of flow speeds using hot-wire anemometry to measure the boundary layer resulting from each tripping method. The results have demonstrated that to produce a natural turbulent boundary layer using a 2D protuberance, the height of the trip must be less than the undisturbed boundary layer thickness. Using such a trip was shown to reduce the development length of the turbulent boundary layer by approximately 50%. This was shown to hold true for all Reynolds numbers investigated ( Re x = 1.2 × 10 5 − 1.5 × 10 6 ) . The present study provides an insight into the effect of the investigated trip techniques on the induced transition of a laminar boundary layer into turbulence.

The phenomenon of relaminarization is observed in many flow situations, including that of an initially turbulent boundary layer (TBL) subjected to strong favorable pressure gradients (FPG). As several experiments on relaminarizing flows have indicated, TBLs subjected to high pressure gradients do not follow the universal log-law, and (for this and other reasons) the prediction of boundary layer (BL) parameters using current turbulence models has not been successful. However, a quasi-laminar theory (QLT; proposed in 1973), based on a two-layer model to explain the later stages of relaminarization, showed good agreement with the experimental data available at that time. These data were mostly at relatively low Re and hence left the precise role of viscosity undefined. QLT, therefore, could not be assessed at high-Re. Recent experiments, however, have provided more comprehensive data and extended the Reynolds number range to nearly 5 × 10 3 in momentum thickness. These data provide a basis for a reassessment of QLT, which is revisited here with an improved predictive code. It is demonstrated that even for these high-Re flows subjected to high acceleration, QLT provides good agreement with experimental results, and therefore, has the potential to substitute for Reynolds-averaged Navier–Stokes (RANS) simulations in high FPG regions.

The transition from a laminar to turbulent boundary layer on a wing operating at low Reynolds numbers can have a large effect on its aerodynamic performance. For a wing operating in ground effect, where very low pressures and large pressure gradients are common, the effect is even greater. A study was conducted into the effect of forcing boundary-layer transition on the suction surface of an inverted GA(W)-1 section single-element wing in ground effect, which is representative of a racing-car front wing. Transition to a turbulent boundary layer was forced at varying chordwise locations and compared to the free-transition case using experimental and computational methods. Forcing transition caused the laminar-separation bubble, which was the unforced transition mechanism, to be eliminated in all cases and trailing-edge separation to occur instead. The aerodynamic forces produced by the wing with trailing-edge separation were shown to be dependent on trip location. As the trip was moved upstream the separation point also moved upstream, this led to an increase in drag and reduction in downforce. In addition to significant changes to the pressure field around the wing, turbulent energy in the wake was considerably reduced by forcing transition. The differences between free- and forced-transition wings were shown to be significant, highlighting the importance of modeling transition for ground-effect wings. Additionally, it has been shown that while it is possible to reproduce the force coefficient of a higher Reynolds-number case by forcing the boundary layer to a turbulent state, the flow features, both on-surface and off-surface, are not recreated.

A time-periodic blowing/suction is provided to control turbulent separation in a boundary layer using direct numerical simulation. The blowing/suction is given just before the separation point, and its nondimensional forcing frequency ranges from F* = fL b /U ∞ = 0.28–8.75, where f is the forcing frequency, L b is the streamwise length of uncontrolled separation bubble, and U ∞ is the freestream velocity. The size of separation bubble is minimum at F* = 0.5. At low forcing frequencies of F* ≤ 0.5, vortices generated by the forcing travel downstream at convection velocity of 0.32–0.35 U ∞ , bring high momentum toward the wall, and reduce the size of separation bubble. However, at high forcing frequencies of F* ≥ 1.56, flow separation disappears and appears in time during the forcing period. This phenomenon occurs due to high wall-pressure gradients alternating favorably and adversely in time. A potential flow theory indicates that this rapid change of the wall pressure in time occurs through an inviscid mechanism. Finally, it is shown that this high-frequency forcing requires a large control input power due to high pressure work.

Experimental observations of towed sonar arrays as characterized by long thin circular cylinders indicate transverse motions that are clearly identified by low-amplitudes, low-wavelengths, and low-frequencies. Although the cylinder length (L) to radius (a) is commonly large [L/a = O(10 3 )] with high Reynolds numbers [O(10 4 )], the corresponding length scale involving the average skin friction [C f L/a = O(10)] remains within the many experimental determinations of short to moderate length cylinders that experience oscillatory instabilities. Prior to the present investigation, any detrimental effects of these oscillatory instabilities on the thin cylinder flow physics that serve construction of the respective semi-empirical and semi-analytical models remained chiefly unknown. Herein, we began examining those turbulent statistics via fine-scale numerical simulations to critique the pragmatic adequacy of the representative design models. We were concerned in particular about the streamwise effects on the turbulent boundary layer (TBL), skin friction and wall pressure evolutions as well as the radial distributions of the leading normal and shear Reynolds stresses. Fortunately, no major deviations (within 10%) were discovered in the TBL statistics over a characteristic range of Reynolds numbers and TBL thicknesses as compared to the axisymmetric state. However, acute spikes (both subharmonics and harmonics) were detected in the wall pressure autospectra similar to that suspected in the towed cylinder experiments, which were conducted in large tow tanks and lake-type basins. These spikes are of paramount importance and should be explored further because they may lead to signal-to-noise ratios above acceptable limits.

A simple variant of recycling and rescaling method to generate inflow turbulence using unstructured grid computational fluid dynamics (CFD) codes is presented. The method has been validated on large eddy simulation (LES) of spatially developing flat plate turbulent boundary layer. The proposed rescaling algorithm is based on the momentum thickness which is more robust and essentially obviates the need of finding the edge of the turbulent boundary layer in unstructured grid codes. Extension of this algorithm to hybrid Reynolds-Averaged Navier-Stokes (RANS) LES type of approaches and for wall-bounded turbomachinery flows is also discussed. Results from annular diffuser with different inflow boundary layer characteristics are presented as an example application to show the utility of such an algorithm.

An experimental investigation was conducted to quantify the characteristics of the turbulent boundary layer flows over a dimpled surface in comparison to those over a conventional flat plate. In addition to measuring surface pressure distributions to determine the friction factors of the test plates and to map the surface pressure inside the dimple cavity, a high-resolution digital particle image velocimetry (PIV) system was used to achieve detailed flow field measurements to quantify the characteristics of the turbulent boundary layer flows over the test plates and the evolution of the unsteady vortex structures inside the dimple cavity at the middle of the dimpled test plate. It was found that the friction factor of the dimpled plate would be about 30–80% higher than that of the flat plate, depending on the Reynolds number of the test cases. In comparison with those over a conventional flat surface, the flow characteristics of the turbulent boundary layer flows over the dimpled surface were found to be much more complicated with much stronger near-wall Reynolds stress and higher turbulence kinetic energy (TKE) levels, especially in the region near the back rims of the dimples. Many interesting flow features over the dimple surface, such as the separation of oncoming boundary layer flow from the dimpled surface when passing over the dimple front rim, the formation and periodic shedding of unsteady Kelvin–Helmholtz vortices in the shear layer over the dimple, the impingement of the high-speed incoming flow onto the back rim of the dimple, and the subsequent generation of strong upwash flow in the boundary flow to promote the turbulent mixing over the dimpled surface, were revealed clearly and quantitatively from the PIV measurement results. The quantitative measurement results are believed to be the first of its nature, which depict a vivid picture about the unique flow features over dimpled surfaces and their correlations with the enhanced heat transfer performance reported in previous studies.

An experimental investigation has been conducted to identify the effects of pressure gradient and surface roughness on turbulent boundary layers. In Part II, smooth- and rough-surface turbulent boundary layers with and without adverse pressure gradient (APG) are presented at a fixed Reynolds number (based on the length of flat plate) of 900,000. Flat-plate boundary layer measurements have been conducted using a single-sensor, hot-wire probe. For smooth surfaces, compared to the zero pressure gradient (ZPG) boundary layer, the APG boundary layer has a higher mean velocity defect throughout the boundary layer and lower friction coefficient. APG decreases the streamwise normal Reynolds stress for y less than 0.4 times the boundary layer thickness and increases it slightly in the outer region. For rough surfaces, APG reduces the roughness effects of increasing the mean velocity defect and normal Reynolds stress for y less than 23 and 28 times the average roughness height, respectively. Consistently, for the same roughness, APG decreases the integrated streamwise turbulent kinetic energy. APG also decreases the roughness effect on the friction coefficient, roughness Reynolds number, and roughness shift. Compared to the ZPG boundary layers, the roughness effects on integral boundary layer parameters—boundary layer thickness and momentum thickness—are weaker under APG. Thus, contrary to the favorable pressure gradient (FPG) in part I, APG reduces the roughness effects on turbulent boundary layers.

The effects of the pressure gradient and surface roughness on turbulent boundary layers have been experimentally investigated. In Part I, smooth- and rough-surface turbulent boundary layers with and without favorable pressure gradients (FPG) are presented. All of the tests have been conducted at the same Reynolds number (based on the length of the flat plate) of 900,000. Streamwise time-mean and fluctuating velocities have been measured using a single-sensor hot-wire probe. For smooth surfaces, the FPG decreases the mean velocity defect and increases the wall shear stress; however, the friction coefficient hardly changes due to the increased freestream velocity. The FPG effect on the streamwise normal Reynolds stress has been examined. The FPG increases the streamwise normal Reynolds stress for y less than 0.6 times the boundary layer thickness. With the zero pressure gradient (ZPG), the roughness increases the mean velocity defect throughout the boundary layer and increases the normal Reynolds stress for y greater than twice the average roughness height. The roughness effect on the mean velocity defect is stronger under the FPG than under the ZPG for y less than 30 times the average roughness height. For y less than 25 times the average roughness height, the roughness effect of increasing normal Reynolds stress is also stronger under the FPG than under the ZPG. Consequently, for a rough surface, the FPG increases the integrated streamwise turbulent kinetic energy, friction coefficient, roughness Reynolds number, and roughness shift. Furthermore, the FPG increases the roughness effects on the integral boundary layer parameters—the boundary layer thickness, momentum thickness, ratio of the displacement thickness to the boundary layer thickness, and shape factor. Thus, the FPG augments the roughness effects on turbulent boundary layers.

Smooth and rough wall turbulent boundary layer profiles are frequently scaled using the wall shear velocity u*, thus it is important that u* is accurately known. This paper reviews and assesses several wall similarity techniques to determine u* and compares results with data from the total stress, Preston tube, and direct force methods. The performance of each method was investigated using experimental repeatability data of smooth and rough wall turbulent boundary layer profiles at Reθ of 3330 and 4840, respectively, obtained using laser Doppler velocimetry (LDV) in a recirculating water tunnel. To validate the results, an analysis was also performed on the direct numerical simulation (DNS) data of Jimenez et al. (2010, “Turbulent Boundary Layers and Channels at Moderate Reynolds Numbers,” J. Fluid Mech., 657, pp. 335–360) at Reθ = 1968. The inner layer similarity methods of Bradshaw had low experimental uncertainty and accurately determined u* and ε for the DNS data and are the recommended wall similarity methods for turbulent boundary layer profile analysis. The outer layer similarity methods did not perform well, due to the need to simultaneously solve for three parameters: u*, ε, and Π. It is strongly recommended that the u* values determined using wall similarity techniques are independently verified using another method such as the total stress or direct force methods.

Even after several decades of experimental and numerical testing, our present-day knowledge of the axisymmetric turbulent boundary layer (TBL) along long thin circular cylinders still lacks a clear picture of many fundamental characteristics. The main issues causing this reside in the experimental testing complexities and the numerical simplifications. An important characteristic that is crucial for routine scaling is the boundary layer length scales, but the downstream growth of these scales (boundary layer, displacement, and momentum thicknesses) is largely unknown from the leading to trailing edges. Herein, we combine pertinent datasets with many complementary numerical computations (large-eddy simulations) to address this shortfall. We are particularly interested in expressing the length scales in terms of the radius-based and axial-based Reynolds numbers (Rea and Rex). Although the composite dataset gave an averaged shape factor H = 1.09 that is substantially lower than the planar value (H = 1.27), the shape factor distribution along the cylinder axis actually begins at the flat plate value then decays logarithmically to near unity. The integral length scales displayed power-law evolutions with variable exponents until high Rea (Rea > 35,000) where both scales then mimic streamwise consistency. Beneath this threshold, their streamwise growth is much slower than the flat plate (especially at low-Rea). The boundary layer thickness grew according to an empirical expression that is dependent on both Rea and Rex where its streamwise growth can far exceed the planar turbulent flow. These unique characteristics rank the thin cylinder axisymmetric TBL as a separate canonical flow, which was well documented by the previous investigations.

Experiments on vortex shedding from a blunt trailing edge symmetric hydrofoil operating at zero angle of attack in a uniform high speed flow, Re h = 16 . 1 · 10 3 - 96 . 6 · 10 3 , where the reference length h is the trailing edge thickness, are reported. The effects of a tripped turbulent boundary layer on the wake characteristics are analyzed and compared with the condition of a natural turbulent transition. The foil surface is hydraulically smooth and a fully effective boundary layer tripping at the leading edge is achieved with the help of a distributed roughness. The vortex shedding process is found to be strongly influenced by the boundary layer development: the tripped turbulent transition promotes the re-establishment of organized vortex shedding. In the context of the tripped transition and in comparison with the natural one, significant increases in the vortex span-wise organization, the vortex-induced hydrofoil vibration, the wake velocity fluctuations, and the vortex strength are revealed. Although the vortex shedding frequency is decreased, a modified Strouhal number based on the wake width at the end of the vortex formation region is constant and evidences the similarity of the wakes in terms of spatial distribution for the two considered boundary layer transition processes.