Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session D26: Surface Roughness: GeneralBoundary Layers

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Chair: Armin Zare, University of Southern California Room: 707 
Sunday, November 19, 2017 2:15PM  2:28PM 
D26.00001: Documentation of rollerbearing effect on butterfly inspired grooves Sashank Gautam, Amy Lang Butterfly wings are covered with scales in a roof shingle pattern which align together to form grooves. The increase or decrease of laminar friction drag depends on the flow orientation to the scales. Flow in the longitudinal direction to the grooves encounters increased surface area which increases the friction drag. However, in the transverse direction, for low Re laminar flow, a single vortex is formed inside each groove and is predicted to remain stable due to the very low Re of the flow in each cavity. These embedded vortices act as roller bearings to the flow above, such that the fluid from the outer boundary layer does not mix with fluid inside the cavities. This leads to a reduction of skin friction drag when compared to a smooth surface. When the cavity flow Re is increased beyond a critical point, the vortex becomes unstable and the lowmomentum fluid in the grooves mixes with the outer boundary layer flow, increasing the drag. The objective of this experiment is to determine the critical Re where the embedded vortex transitions from a stable to an unstable state using DPIV. Subsequently, for steady vortex conditions, a comparison of skin friction drag between the grooved and flat plate can show that the butterfly scaled surface can result in sublaminar friction drag. [Preview Abstract] 
Sunday, November 19, 2017 2:28PM  2:41PM 
D26.00002: The Influence of Roof Material on Diurnal Urban Canyon Breathing Mohamed Abuhegazy, Neda Yaghoobian Improvements in building energy use, air quality in urban canyons and in general urban microclimates require understanding the complex interaction between urban morphology, materials, climate, and inflow conditions. Review of the literature indicates that despite a long history of valuable urban microclimate studies, more comprehensive approaches are needed to address energy, and heat and flow transport in urban areas. In this study, a more comprehensive simulation of the diurnally varying street canyon flow and associated heat transport is numerically investigated, using Largeeddy Simulation (LES). We use computational modeling to examine the impact of diurnal variation of the heat fluxes from urban surfaces on the air flow and temperature distribution in street canyons with a focus on the role of roof materials and their temperature footprints. A detailed building energy model with a threedimensional rastertype geometry provides urban surface heat fluxes as thermal boundary conditions for the LES to determine the key aerothermodynamic factors that affect urban street ventilation. [Preview Abstract] 
Sunday, November 19, 2017 2:41PM  2:54PM 
D26.00003: Flat Plate Boundary Layer Stimulation Using Trip Wires and Hama Strips. Charles Peguero, Charles Henoch, James Hrubes, Albert Fredette, Raymond Roberts, Stephen Huyer Water tunnel experiments on a flat plate at zero angle of attack were performed to investigate the effect of single roughness elements, i.e., trip wires and Hama strips, on the transition to turbulence. Boundary layer trips are traditionally used in scale model testing to force a boundary layer to transition from laminar to turbulent flow at a single location to aid in scaling of flow characteristics. Several investigations of trip wire effects exist in the literature, but there is a dearth of information regarding the influence of Hama strips on the flat plate boundary layer. The intent of this investigation is to better understand the effects of boundary layer trips, particularly Hama strips, and to investigate the pressureinduced drag of both styles of boundary layer trips. Untripped and tripped boundary layers along a flat plate at a range of flow speeds were characterized with multiple diagnostic measurements in the NUWC/Newport 12inch water tunnel. A wide range of Hama strip and wire trip thicknesses were used. Measurements included dye flow visualization, direct skin friction and parasitic drag force, boundary layer profiles using LDV, wall shear stress fluctuations using hot film anemometry, and streamwise pressure gradients. Test results will be compared to the CFD and boundary layer model results as well as the existing body of work. Conclusions, resulting in guidance for application of Hama strips in model scale experiments and nondimensional predictions of pressure drag will be presented. [Preview Abstract] 
Sunday, November 19, 2017 2:54PM  3:07PM 
D26.00004: Steady Boundary Layer Disturbances Created By TwoDimensional Surface Ripples Matthew Kuester Multiple experiments have shown that surface roughness can enhance the growth of TollmienSchlichting (TS) waves in a laminar boundary layer. One of the common observations from these studies is a ``wall displacement'' effect, where the boundary layer profile shape remains relatively unchanged, but the origin of the profile pushes away from the wall. The objective of this work is to calculate the steady velocity field (including this wall displacement) of a laminar boundary layer over a surface with small, 2D surface ripples. The velocity field is a combination of a Blasius boundary layer and multiple disturbance modes, calculated using the linearized NavierStokes equations. The method of multiple scales is used to include nonparallel boundary layer effects of $\mathcal{O}$$(R_{\delta}^{1})$; the nonparallel terms are necessary, because a wall displacement is mathematically inconsistent with a parallel boundary layer assumption. This technique is used to calculate the steady velocity field over ripples of varying height and wavelength, including cases where a separation bubble forms on the leeward side of the ripple. In future work, the steady velocity field will be the input for stability calculations, which will quantify the growth of TS waves over rough surfaces. [Preview Abstract] 
Sunday, November 19, 2017 3:07PM  3:20PM 
D26.00005: On the dynamics of the flow in the vicinity of microscale coatings composed by organized elements Ali Doosttalab, Humberto Bocanegra Evans, Serdar Gorumlu, Burak Aksak, Leonardo P. Chamorro, Luciano Castillo A set of highresolution PIV experiments were carried out in a refractive indexmatched facility under zero pressure gradient turbulent boundary layer to investigate the flow dynamics around two customized coatings composed of uniformly distributed fibers of different geometry. The two type of fibers shared a cylindrical shape and height $y^+<1$; however, one of those had diverging tip similar to that of a shark skin. Results evidence an interlayer acting between the viscousdominated flow within the pillars canopy (where $Re \sim 1$) and the inertia dominated flow in the boundary layer. Using averaged 2D NS equations, it is possible to show that the interlayer wall shear stress is $\tau_{oiw}^+ = [\frac{\partial{U^+}}{\partial{y^+}}  \langle{}uv^+\rangle] [P_w^+h^+ (\frac{y^+}{h^+}  1) +\langle{}u_{oi}v_{oi}^+\rangle ]$, with first term in the RHS representing the wall shear stress and the second term indicating the interlayer form drag. A wallnormal Reynolds stress exist which depends on the pressure difference across the boundary layer and at the wall, $\langle{}v^2\rangle = \langle{}v_{oi}^2\rangle + (\langle P_{w} \rangle  \langle P \rangle)/\rho$. This reveals a basic mechanism where the flow is modulated by unsteady blowing and suction at the interface. [Preview Abstract] 
Sunday, November 19, 2017 3:20PM  3:33PM 
D26.00006: ABSTRACT WITHDRAWN 
Sunday, November 19, 2017 3:33PM  3:46PM 
D26.00007: Nonlinear Effects of Surface Roughness on Stationary Crossflow Vortices in ThreeDimensional Boundary Layers Adam Butler, Xuesong Wu HighReynoldsnumber 3D boundarylayer flows are strongly dependent on external disturbances. The behaviour of the stationary crossflow instability, which is typically the dominant instability on swept wings in flight conditions, is heavily influenced by surface roughness. We investigate nonlinear interactions between roughness modes and upperbranch stationary crossflow modes when their critical layers coincide. In particular, we focus on the case when the roughness consists of a single mode with slowlyvarying amplitude and chordwise and spanwise wavenumbers that are an integer multiple of the crossflow wavenumbers, $\left(\alpha_{w},\beta_{w}\right)=r\left(\alpha,\beta\right)$. In general, the two modes interact at the cubic order to drive jump conditions across the critical layer and influence both the linear and nonlinear development of the crossflow mode. Importantly, this interaction occurs for small surface roughness: in terms of the Reynolds number $R$ based on the local boundarylayer thickness $\delta^{*}$, this interaction takes place for surface roughness of height $h^{*}\sim\delta^{*}R^{\frac{1}{2}}$. The nonlinear amplitude equation is derived and solved numerically, and the importance of these results for the downstream development of the flow is discussed. [Preview Abstract] 
Sunday, November 19, 2017 3:46PM  3:59PM 
D26.00008: Characteristics of turbulent boundary layers over smooth surfaces with spanwise heterogeneities. Takfarinas Medjnoun, Christina Vanderwel, Bharathram Ganapathisubramani An experimental investigation of a turbulent boundary layer flow over an idealized rough surface is performed to examine the mean and turbulent flow characteristics, but also to document the variation of skinfriction that might affect the applicability of traditional scaling and similarity laws. These spanwise heterogeneous surfaces whose lengthscale is comparable to the boundary layer thickness are known to create largescale secondary motions. Singlepoint velocity measurements combined with direct skinfriction measurements are used to examine Townsend's hypothesis. The results reveal a drag increase of 35\% relative to the smooth wall. Additionally, they show the existence of a loglayer in the mean velocity profile with a zeroplane displacement and a roughness function that vary across the span. Lack of collapse in the outer region of the mean velocity and variance profiles is attributed to the secondary flows induced by the heterogeneous surfaces. The lack of similarity also extends to the spectra across all scales in the nearwall region with a gradual collapse at low wavelengths for increasing spacing. This suggests that the effect of secondary flows is not necessarily felt at the smaller scales other than to reorganize their presence through turbulent transport. [Preview Abstract] 
Sunday, November 19, 2017 3:59PM  4:12PM 
D26.00009: Towards predictive models for transitionally rough surfaces Nabil AbderrahamanElena, Ricardo GarciaMayoral We analyze and model the previously presented decomposition for flow variables in DNS of turbulence over transitionally rough surfaces. The flow is decomposed into two contributions: one produced by the overlying turbulence, which has no footprint of the surface texture, and one induced by the roughness, which is essentially the timeaveraged flow around the surface obstacles, but modulated in amplitude by the first component. The roughnessinduced component closely resembles the laminar steady flow around the roughness elements at the same nondimensional roughness size. For small yet transitionally rough textures, the roughnessfree component is essentially the same as over a smooth wall. Based on these findings, we propose predictive models for the onset of the transitionally rough regime. [Preview Abstract] 
Sunday, November 19, 2017 4:12PM  4:25PM 
D26.00010: Critical surface roughness for wall bounded flow of viscous fluids in an electric submersible pump Dhairyasheel Deshmukh, md hamid Siddique, Frank Kenyery, Abdus Samad Surface roughness plays a vital role in the performance of an electric submersible pump (ESP). A 3D numerical analysis has been carried out to find the roughness effect on ESP. The performance of pump for steady wall bounded turbulent flows is evaluated at different roughness values and compared with smooth surface considering a nondimensional roughness factor K. The kω SST turbulence model with fine mesh at near wall region captures the rough wall effects accurately. Computational results are validated with experimental results of water (1 cP), at a design speed (3000 RPM). Maximum head is observed for a hydraulically smooth surface (K=0). When roughness factor is increased, the head decreases till critical roughness factor (K=0.1) due to frictional loss. Further increase in roughness factor (K>0.1) increases the head due to near wall turbulence. The performance of ESP is analyzed for turbulent kinetic energy and eddy viscosity at different roughness values. The wall disturbance over the rough surface affects the pressure distribution and velocity field. The roughness effect is predominant for high viscosity oil (43cP) as compared to water. Moreover, the study at offdesign conditions showed that Reynolds number influences the overall roughness effect. [Preview Abstract] 
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