PIV Measurements of the Cross-Flow Velocity Field in the Near Wake of a Generic Pickup Truck

2006 ◽  
Author(s):  
Bahram Khalighi
Keyword(s):  
Velocity Field ◽  
Symmetry Plane ◽  
Cross Flow ◽  
Velocity Data ◽  
Near Wake ◽  
Mean Velocity ◽  
Piv Measurements ◽  
Pickup Truck ◽  
The Mean ◽  
The Cross

The cross-flow field (flow in planes normal to the direction of motion) in the near wake of a generic pickup truck is investigated experimentally using Particle Image Velocimetry (PIV). The PIV measurements of the velocity field normal to the free-stream direction are carried out at four stream-wise locations behind the cab and the tailgate. The PIV data are processed to obtain the instantaneous velocity field, the mean and the turbulence properties of the flow. The instantaneous results in the near wake of the cab show various vortical structures. The mean velocity data shows that the flow moves from the sides toward the center of the bed near the tailgate. The velocity data in the near wake of the tailgate shows a pair of counter-rotating vortices that induces a downwash velocity field at the symmetry plane. This downwash promotes an attached flow behind the tailgate, thus generating a pressure recovery which leads to reductions in the total drag.

Aerodynamic Analysis of Pickup Truck

2011 ◽  
Vol 201-203 ◽  
pp. 1296-1299
Author(s):  
Xiao Ni Qi ◽  
Jian Meng ◽  
Yong Qi Liu
Keyword(s):  
Reverse Flow ◽  
Symmetry Plane ◽  
Cross Flow ◽  
Flow Region ◽  
Flow Fields ◽  
Mean Flow ◽  
Near Wake ◽  
Recirculation Flow ◽  
Pickup Truck ◽  
The Cross

The present study focuses on the aerodynamics of pickup trucks. The CFD software FLUENT was used to simulate flow field around a pickup truck in this paper. Numerical simulation was taken on a 1/5th pickup truck model. The surface pressure distribution, the wake velocity distribution of the special profiles and the flow structures were obtained. The research indicted that there was a recirculation flow region over the bed for pickup truck. The cab shear layer did not interact directly with the tailgate, flowing above the top of the tailgate. There was a downwash flow in the symmetry plane behind the tailgate with no reverse flow region in the symmetry plane, and the formation of two smaller recirculation flow regions was on both sides of the symmetry plane. Mean flow fields in the near wake of the cab showed a weak pair of counter-rotating vortices behind the cab. In the cross-flow planes behind the tailgate, the mean flow fields show strong counter-rotating vortices behind the tailgate. Instantaneous flow fields in the cross-flow planes of the pickup truck near wake showed compact vortex structures located randomly in space.

Three-dimensional structure and momentum transfer in a turbulent cylinder wake

1999 ◽  
Vol 394 ◽  
pp. 303-337 ◽  
Author(s):  
A. VERNET ◽  
G. A. KOPP ◽  
J. A. FERRÉ ◽  
FRANCESC GIRALT
Keyword(s):  
Velocity Field ◽  
Saddle Point ◽  
Three Dimensional ◽  
Symmetry Plane ◽  
Half Width ◽  
Small Scale ◽  
Spanwise Direction ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean

Simultaneous velocity and temperature measurements were made with rakes of sensors that sliced a slightly heated turbulent wake in the spanwise direction, at different lateral positions 150 diameters downstream of the cylinder. A pattern recognition analysis of hotter-to-colder transitions was performed on temperature data measured at the mean velocity half-width. The velocity data from the different ‘slices’ was then conditionally averaged based on the identified temperature events. This procedure yielded the topology of the average three-dimensional large-scale structure which was visualized with iso-surfaces of negative values of the second eigenvector of [S2+Ω2]. The results indicate that the average structure of the velocity fluctuations (using a triple decomposition of the velocity field) is found to be a shear-aligned ring-shaped vortex. This vortex ring has strong outward lateral velocities in its symmetry plane which are like Grant's mixing jets. The mixing jet region extends outside the ring-like vortex and is bounded by two foci separated in the spanwise direction and an upstream saddle point. The two foci correspond to what has been previously identified in the literature as the double rollers.The ring vortex extracts energy from the mean flow by stretching in the mixing jet region just upstream of the ring boundary. The production of the small-scale (incoherent) turbulence by the coherent field and one-component energy dissipation rate occur just downstream of the saddle point within the mixing jet region. Incoherent turbulence energy is extracted from the mean flow just outside the mixing jet region, but within the core of the structure. These processes are highly three-dimensional with a spanwise extent equal to the mean velocity half-width.When a double decomposition is used, the coherent structure is found to be a tube-shaped vortex with a spanwise extent of about 2.5l0. The double roller motions are integral to this vortex in spite of its shape. Spatial averages of the coherent velocity field indicate that the mixing jet region causes a deficit of mean streamwise momentum, while the region outside the foci of the double rollers has a relatively small excess of streamwise momentum.

scholarly journals Global energy fluxes in turbulent channels with flow control

2018 ◽  
Vol 857 ◽  
pp. 345-373 ◽  
Author(s):  
Davide Gatti ◽  
Andrea Cimarelli ◽  
Yosuke Hasegawa ◽  
Bettina Frohnapfel ◽  
Maurizio Quadrio
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Energy Dissipation ◽  
Velocity Field ◽  
Flow Control ◽  
Reynolds Shear Stress ◽  
Power Input ◽  
Energy Fluxes ◽  
Mean Velocity ◽  
The Mean

This paper addresses the integral energy fluxes in natural and controlled turbulent channel flows, where active skin-friction drag reduction techniques allow a more efficient use of the available power. We study whether the increased efficiency shows any general trend in how energy is dissipated by the mean velocity field (mean dissipation) and by the fluctuating velocity field (turbulent dissipation). Direct numerical simulations (DNS) of different control strategies are performed at constant power input (CPI), so that at statistical equilibrium, each flow (either uncontrolled or controlled by different means) has the same power input, hence the same global energy flux and, by definition, the same total energy dissipation rate. The simulations reveal that changes in mean and turbulent energy dissipation rates can be of either sign in a successfully controlled flow. A quantitative description of these changes is made possible by a new decomposition of the total dissipation, stemming from an extended Reynolds decomposition, where the mean velocity is split into a laminar component and a deviation from it. Thanks to the analytical expressions of the laminar quantities, exact relationships are derived that link the achieved flow rate increase and all energy fluxes in the flow system with two wall-normal integrals of the Reynolds shear stress and the Reynolds number. The dependence of the energy fluxes on the Reynolds number is elucidated with a simple model in which the control-dependent changes of the Reynolds shear stress are accounted for via a modification of the mean velocity profile. The physical meaning of the energy fluxes stemming from the new decomposition unveils their inter-relations and connection to flow control, so that a clear target for flow control can be identified.

scholarly journals A Novel Simplification of the Reynolds-Chou-Navier-Stokes Turbulence Equations of Incompressible Flow

2018 ◽  
Author(s):  
Bohua Sun
Keyword(s):  
Velocity Field ◽  
Stress Tensor ◽  
Reynolds Stress ◽  
Incompressible Flow ◽  
Velocity Fluctuation ◽  
Explicit Representation ◽  
Navier Stokes ◽  
Mean Velocity ◽  
The Mean ◽  
Turbulence Formulations

Based on author's previous work [Sun, B. The Reynolds Navier-Stokes Turbulence Equations of Incompressible Flow Are Closed Rather Than Unclosed. Preprints 2018, 2018060461 (doi: 10.20944/preprints201806.0461.v1)], this paper proposed an explicit representation of velocity fluctuation and formulated the Reynolds stress tensor in terms of the mean velocity field. The proposed closed Reynolds Navier-Stokes turbulence formulations reveal that the mean vorticity is the key source of producing turbulence.

Effects of vortex-induced velocity on the development of a synthetic jet issuing into a turbulent boundary layer

2019 ◽  
Vol 870 ◽  
pp. 651-679 ◽  
Author(s):  
Tim Berk ◽  
Bharathram Ganapathisubramani
Keyword(s):  
Velocity Field ◽  
Momentum Transfer ◽  
Momentum Flux ◽  
Cross Flow ◽  
Measured Data ◽  
Velocity Fields ◽  
Synthetic Jet ◽  
Vortical Structures ◽  
The Cross ◽  
Induced Velocity

A synthetic jet issuing into a cross-flow influences the local velocity of the cross-flow. At the jet exit the jet is oriented in the wall-normal direction while the cross-flow is oriented in the streamwise direction, leading to a momentum transfer between the jet and the cross-flow. Streamwise momentum transferred from the cross-flow to the jet accelerates the pulses created by the jet. This momentum transfer continuous up to some point downstream where these pulses have the same velocity as the surrounding flow and are no longer blocking the cross-flow. The momentum transfer from the cross-flow to the jet leads to a momentum deficit in the cross-flow far downstream of the viscous near field of the jet. In the literature this momentum-flux deficit is often attributed to viscous blockage or to up-wash of low-momentum fluid. The present paper proposes and quantifies a third source of momentum deficit: a velocity induced opposite to the cross-flow by the vortical structures created by the synthetic jet. These vortical structures are reconstructed from measured data and their induced velocity is calculated using the Biot–Savart law. The three-dimensional three-component induced velocity fields show great similarity to the measured velocity fields, suggesting that this induced velocity is the main contributor to the velocity field around the synthetic jet and viscous effects have only a small influence. The momentum-flux deficit induced by the vortical structures is compared to the measured momentum-flux deficit, showing that the main part of this deficit is caused by the induced velocity. Variations with Strouhal number (frequency of the jet) and velocity ratio (velocity of the jet) are observed and discussed. An inviscid-flow model is developed, which represents the downstream evolution of the jet in cross-flow. Using the measured data as an input, this model is able to predict the deformation, (wall-normal) evolution and qualitative velocity field of the jet. The present study presents evidence that the velocity induced by the vortical structures forming a synthetic jet plays an important role in the development of and the velocity field around the jet.

PIV Measurements in a Confined Jet

2002 ◽  
Author(s):  
A. Shinneeb ◽  
J. D. Bugg ◽  
R. Balachandar
Keyword(s):  
Free Surface ◽  
Velocity Field ◽  
Surface Flow ◽  
Recirculation Region ◽  
Maximum Deviation ◽  
Centerline Velocity ◽  
Upward Flow ◽  
Piv Measurements ◽  
Exit Velocity ◽  
The Mean

This paper reports PIV measurements made at three locations in an axisymmetric, confined jet that is approaching a free surface from below. The apparatus consists of a tank 40.5 cm × 40.5 cm at its base and 61 cm high. A 9 mm diameter nozzle is centered in the base of the tank and directs a jet of water upwards. The jet produced has a top-hat velocity profile with a maximum deviation of 0.32% of the mean and an axial relative turbulence intensity of 0.60%. The water is removed from the tank by an overflow around the perimeter of the tank. The PIV measurements achieved a spatial resolution of between 0.425–1.08 mm. The measurements show details of the velocity field in three regions of the flow; at the jet exit, near the surface on the centerline of the jet, and near the top corner of the tank. The centerline velocity remains at the exit velocity until ≈5D from the exit. The axial confinement of the jet begins to significantly influence the centerline velocity at ≈13D from the free surface. All entrained fluid is deflected downward from the horizontal surface flow as it approaches the overflow around the perimeter of the tank. This creates a large recirculation region in the upper region of the tank driven by the downward flow along the wall and the upward flow of the jet itself at the center of the tank.

Measurements of the Velocity Field Downstream of an Impeller

1996 ◽  
Vol 118 (3) ◽  
pp. 602-610 ◽  
Author(s):  
Per Petersson ◽  
Magnus Larson ◽  
Lennart Jo¨nsson
Keyword(s):  
Velocity Field ◽  
Momentum Flux ◽  
Laser Doppler Velocimeter ◽  
Self Similarity ◽  
Mean Velocity ◽  
Measured Velocity ◽  
Swirling Jets ◽  
Axial Velocity Profile ◽  
Two Component ◽  
The Mean

The velocity field downstream of a model impeller operating in water was measured using a two-component laser doppler velocimeter. The investigation focussed on the spatial development of the mean velocity in the axial, radial, and circumferential direction, although simultaneous measurements were performed of the velocity unsteadiness from which turbulence characteristics were inferred. The measurements extended up to 12 impeller diameters downstream of the blades displaying the properties of the generated swirling jet both in the zone of flow establishment and the zone of established flow. The division between these zones was made based on similarity of the mean axial velocity profile. Integral properties of the flow such as volume and momentum flux were computed from the measured velocity profiles. The transverse spreading of the impeller jet and its development towards self-similarity were examined and compared with non-swirling jets and swirling jets generated by other means.

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