Comparison of turbulence modeling approaches to the simulation of a dimpled sphere

Use of computational fluid dynamics (CFD) in the aerodynamic simulation of sports projectiles has always been a challenge. The majority of these are spherical, classic bluff bodies, which typically experience flow transition during flight, and large flow separations. Current research of such flows is predominantly concentrated on the use of computationally intensive large eddy scale (LES) simulation methods, and even direct numerical simulation (DNS). Use of such approaches requires careful application of the models, and significant computational resource. The alternative is the use of unsteady Reynolds-averaged Navier Stokes (URANS) turbulence models, which are typically known to struggle in such flow scenarios. URANS however are, in comparison to LES, computationally economical and as such these models find significant use amongst both industry and academia alike, and their development still continues. In recent years transitional URANS models based on the calculation of intermittency, and hybrid scale resolving simulation approaches (SRS), have started to appear in proprietary CFD codes. Hybrid SRS models such as scale adaptive simulation (SAS) and detached eddy simulation (DES), combine LES with the use of economical well tuned URANS in the simulation of near wall flows. However to date the use of such models in the simulation of sports projectiles has been extremely limited. This paper provides a CFD comparison of these turbulence modelling approaches, with application to the simulation of a dimpled sphere, a golf ball. The study investigates and compares the suitability of URANS, transitional URANS, and SRS models. Simulations are run between 10,000 < Re < 115,000, from sub-critical through transition to supercritical. Comparisons are drawn between predictions of drag coefficient, dimple shear layers and surface shear stress, with URANS being shown to be in reasonable agreement with SRS. However as may be expected although URANS predicted a comparable size of wake to SRS, no small scale structure was observed. Indeed it is shown how URANS failed to demonstrate any large scale time periodic shedding phenomena, instead becoming essentially steady state

Aerodynamic characteristics and flow pattern of a golf ball with rotation

AbstractThe present study investigated the aerodynamic characteristics and flow pattern of a golf ball with 328 circular arc dimples. The aerodynamic forces and the flow pattern around a golf ball were investigated experimentally. The detailed flow pattern around a golf ball was then investigated computationally by Large eddy simulation. The aerodynamic characteristics and the flow pattern of a golf ball with rotation were clarified. In addition, the flying distance was clarified through a flying simulation based on the aerodynamic forces and the flow pattern

The evaluation of lift and drag force for solid body with micro dimple

The present study explores the lift and drag mechanism, pressure distribution along the chord and flow visualization of the debossed dimpled- aerofoil with the aim to evaluation the effect of dimples on the aerodynamic characteristic on the aerofoil and find an optimisation on the dimples parameter in improving the aerodynamic performance. The issues on the fuel consumption, energy cost, aerodynamic performance efficiency for aerofoil application in turbine, automotive especially under unexpected weather condition such as air turbulence in air flight lead to the concern on improving the aerodynamic performance from time to time. In current context, the interaction of the dimple parameters influencing the aerodynamic behaviour of the aerofoil such as the dimple aspect ratio, shape, pitch resulting in variation of number of dimple is considered. An aerofoil sized 0.14m x 0.148m is tested under wind tunnel with different angle of attack and air velocity. The aerofoil flow visualizations under smoke test are captured with a high speed camera. A comparative study of the smooth and dimpled aerofoil with cylindrical (bluff) and hexagonal (blunt) dimple shape is investigated. From the result it is shown with the integration of dimple, the lift performance of the aerofoil is greatly improved with the delay of flow separation as a result from the streamwise vorticity reduces the turbulent skin drag. thus increasing the lift and the controllability of the aerofoil. The cylindrical dimpled aerofoil with the aspect ratio of 0.4 and pitch 8500 μm is highly recommended with the highest lift and lowest drag

Adapting Shot Peening for Surface Texturing Using Customized Additive Manufactured Shots

Surface textures in engineering materials not only affect the reflective properties and aesthetics but if properly designed can modulate surface-related properties such as wettability, fatigue, wear, corrosion, and scratch resistance. Herein, a new surface texturing method is introduced based on the conventional shot peening process. Custom shots are designed, and their surface texturing capability is investigated on acrylonitrile butadiene styrene (ABS) polymer substrates. A finite-element model is developed to bombard the substrate using AISI 316 stainless steel customized shots. The generated unique textures are compared qualitatively by visual examination and quantitatively using the standard surface roughness parameters. As a proof of concept, preliminary experiments are performed using a candidate custom shot and a spherical shot to treat the ABS sheets. The results highlight the high potential of the shot peening technique paired with additive manufacturing for customizing the peening media to be used for surface texturing polymeric materials

Effect of Dimple Pattern on the Suppression of Boundary Layer Separation on a Low Pressure Turbine Blade

Three dimple patterns were investigated to ascertain their relative effectiveness on controlling boundary layer separation from a low-pressure turbine blade. The three cases included a single row of dimples at 65% of the axial chord with 2.22 cm spacing, a single row of dimples at 65% of the axial chord with 4.44 cm spacing, and a two-row staggered pattern with rows at 65% and 76% of the axial chord with 4.44 cm spacing. The multiple row case was such that the center of the upstream dimple set at the midpoint between two downstream dimples. The dimple spacing was measured center-on-center. Each of the dimple patterns was studied and compared to an unmodified blade at axial chord Reynolds numbers based on inlet velocity of 25k, 45k, and 100k. Experimental data was collected in a low-speed, draw down wind tunnel containing a linear turbine cascade of 8 Pak-B blades. Measurements of surface pressure, boundary layer parameters, wake velocity, and total pressure losses were made to examine the flow. No dimple pattern dramatically outperformed the others. Each of the dimple patterns studied improved the average total pressure loss coefficient by 34% for Re 25k and 1% Tu. Complementing the experimental effort was a three-dimensional computational fluid dynamics study. Four models were built and analyzed. The models included an unmodified blade, blades with dimples at 65% of the axial chord with 2 cm or 4 cm spacing, respectively, and a multiple row case consisting of dimples at 65% and 76% of the axial chord with 2 cm spacing. Again the upstream dimple set at the midpoint between two downstream dimples. The computational fluid dynamics study provided detailed flow visualization in and around the dimples as well as a comparison to experimental data for solver verification. It was shown that the computational and experimental results showed similar trends in wake loss and boundary layer traverses

Conceptual Study of Rotary-Wing Microrobotics

This thesis presents a novel rotary-wing micro-electro-mechanical systems (MEMS) robot design. Two MEMS wing designs were designed, fabricated and tested including one that possesses features conducive to insect level aerodynamics. Two methods for fabricating an angled wing were also attempted with photoresist and CrystalBond™ to create an angle of attack. One particular design consisted of the wing designs mounted on a gear which are driven by MEMS actuators. MEMS comb drive actuators were analyzed, simulated and tested as a feasible drive system. The comb drive resonators were also designed orthogonally which successfully rotated a gear without wings. With wings attached to the gear, orthogonal MEMS thermal actuators demonstrated wing rotation with limited success. Multi-disciplinary theoretical expressions were formulated to account for necessary mechanical force, allowable mass for lift, and electrical power requirements. The robot design did not achieve flight, but the small pieces presented in this research with minor modifications are promising for a potential complete robot design under 1 cm2 wingspan. The complete robot design would work best in a symmetrical quad-rotor configuration for simpler maneuverability and control. The military’s method to gather surveillance, reconnaissance and intelligence could be transformed given a MEMS rotary-wing robot’s diminutive size and multi-role capabilities

Dynamical separation of spherical bodies in supersonic flow

An experimental and computational investigation of the unsteady separation behaviour of two spheres in Mach-4 flow is carried out. The spherical bodies, initially contiguous, are released with negligible relative velocity and thereafter fly freely according to the aerodynamic forces experienced. In experiments performed in a supersonic Ludwieg tube, nylon spheres are initially suspended in the test section by weak threads which are detached by the arrival of the flow. The subsequent sphere motions and unsteady flow structures are recorded using high-speed (13 kHz) focused shadowgraphy. The qualitative separation behaviour and the final lateral velocity of the smaller sphere are found to vary strongly with both the radius ratio and the initial alignment angle of the two spheres. More disparate radii and initial configurations in which the smaller sphere centre lies downstream of the larger sphere centre each increases the tendency for the smaller sphere to be entrained within the flow region bounded by the bow shock of the larger body, rather than expelled from this region. At a critical angle for a given radius ratio (or a critical radius ratio for a given angle), transition from entrainment to expulsion occurs; at this critical value, the final lateral velocity is close to maximum due to the same ‘surfing’ effect noted by Laurence & Deiterding (J. Fluid Mech., vol. 676, 2011, pp. 396–431) at hypersonic Mach numbers. A visualization-based tracking algorithm is used to provide quantitative comparisons between the experiments and high-resolution inviscid numerical simulations, with generally favourable agreement

Use of Dimples to Suppress Boundary Layer Separation on a Low Pressure Turbine Blade

Flow separation on a low pressure turbine blade is explored at Reynolds numbers of 25k, 45k and 100k. Experimental data is collected in a low-speed, draw-down wind tunnel using a cascade of eight Pak-B blades. Flow is examined from measurements of blade surface pressures, boundary layer parameters, exit velocities, and total pressure losses across the blade. Two recessed dimple shapes are assessed for suppressing flow separation and associated losses. One dimple is spherical, and the second is asymmetric, formed from a full dimple spanwise half-filled. A single row of each dimple shape is tested at 50%, 55% and 65% axial chord. Symmetric dimples reduce separation losses by as much as 28%, while asymmetric dimples reduce losses by as much as 23%. A complementary three-dimensional computational study is conducted to visualize local flow structure. Computational analysis uses Gridgen v13.3 as a mesh generator, Fluent® v6.0 as a flow solver and FIELDVIEW© v8.0 for graphic display and analysis. Computational results for Pak-B blades at a Reynolds number of 25k indicate that both dimple shapes cause a span-wise vortex to rollup within the dimple and provide a localized pressure drop

The evaluation of drag and lift force of groove cylinder in wind tunnel

Cylindrical structures subjected to flow are widely used in marine and offshore engineering and in structural applications. It has been shown that circular cylinder surface modifications can affect the separation point to move backward on the cylinder surface. Thus, reducing the drag coefficient. Flow past a circular cylinder with smooth, half and full rectangular, grooved surfaces (roughness coefficient k/D=0.04) were investigated in a low-speed open ended wind tunnel. The outer diameters (D) of the cylinders were 50mm and the depths (k) of the grooves were 2mm. The Reynold's number ranged from 1.65x104 to 1.13x105. The drag and lift coefficients of the cylinders were measured using a three-component balance. The wake flow patterns of the cylinders were observed using a smoke visualization technique. The results show that the full grooves and half grooves facing the flow produced a lower drag than a smooth cylinder with a half groove located at the leeside of the flow. The full grooved cylinder showed a drag reduction of 55% at Re=1x105. The presence of the groove on the cylinder's surface tripped the boundary layer and showed a smaller and narrower wake than a smooth cylinder