218 research outputs found
Drag reduction in a turbulent boundary layer with sinusoidal riblets
Get PDFWe report on an experimental investigation on the effect of sinusoidal riblets on the near-wall characteristics of a turbulent boundary layer. The investigated riblets are characterized by a fixed wavelength and two different values of the amplitude. We comment on the flow field organization via hot wire anemometry, planar and stereoscopic particle image velocimetry experiments; furthermore, we infer on the friction drag, directly measured with a load cell, comparing the sinusoidal riblets to the reference case of riblets aligned with the mean flow (longitudinal riblets) and the Smooth case.We show that the sinusoidal riblets generally yield higher drag reduction, attaining values as large as 10%, compared with the longitudinal riblets that are limited to 8% under the same conditions. We demonstrate that the drag reduction is associated with an overall attenuation of the turbulence intensity in the buffer layer. Furthermore, we provide statistical evidence of the fact that the sinusoidal riblets are responsible for an attenuation of the Reynolds shear stresses that contribute the most to turbulence production. From the detection of the accelerated events in the buffer layer, we show that the sinusoidal riblets lead to a weakening of the intensity of the events in the streamwise plane and an enhancement of the spanwise induced motion. We relate this mechanism to that responsible for drag reduction when using spanwise wall oscillations, suggesting a possible effect of a secondary alternating vorticity in the grooves of the sinusoidal riblets
Flow regimes in a trapped vortex cell
No full textThis paper presents results of an experimental investigation on the flow in a trapped vortex cell, embedded into a flat plate, and interacting with a zero-pressure-gradient boundary layer. The objective of the work is to describe the flow features and elucidate some of the governing physical mechanisms, in the light of recent investigations on flow separation control using vortex cells. Hot-wire velocity measurements of the shear layer bounding the cell and of the boundary layers upstream and downstream are reported, together with spectral and correlation analyses of wall-pressure fluctuation measurements. Smoke flow visualisations provide qualitative insight into some relevant features of the internal flow, namely a large-scale flow unsteadiness and possible mechanisms driving the rotation of the vortex core. Results are presented for two very different regimes: a low-Reynolds-number case where the incoming boundary layer is laminar and its momentum thickness is small compared to the cell opening, and a moderately high-Reynolds-number case, where the incoming boundary layer is turbulent and the ratio between the momentum thickness and the opening length is significantly larger than in the first case. Implications of the present findings to flow control applications of trapped vortex cells are also discussed
Wall bounded flows manipulation using sinusoidal riblets
Get PDFWe experimentally investigate the effects of microgrooves on the development of a zero pressure gradient turbulent boundary layer. Starting from the well-known streamwise aligned riblets, we look at the effect of wavy riblets, characterized by a sinusoidal pattern in the mean flow direction. We perform hot wire experiments as well as particle image velocimetry to get some insights on the effect of the sinusoidal shape on the near wall organisation of the boundary layer. The statistical analysis clearly shows that the wavy pattern has a strong influence on the near wall structure of the boundary layer. The statistical analysis performed using the VITA technique reveals that the coherent structures that characterize the turbulent boundary layer are attenuated by the geometry manipulation. Furthermore, the POD reconstructed velocity fields, measured with PIV, reveal that the manipulation tampers with the momentum exchange occurring between the near wall and the outer region of the boundary layer, hence suggesting a modified turbulence production cycle
Wind tunnel investigation on a squaredâback commercial vehicle
Get PDFThe assessment of an appropriate methodology for a wind tunnel investigation on a square back vehicle model is presented and discussed. The CDâRe curve is evaluated in order
to identify the adequate range of Reynolds number for the tests. The drag coefficient value is attested at CD = 0.465 which is in accordance with the literature results for the same category of
vehicles considered. The repeatability and the accuracy of the CD have been analyzed through a campaign of measurements and an error propagation analysis is dedicated to associate the
correct uncertainty to the drag coefficient values. The analysis of statistical convergence of the measured quantities is also carried out in order to fix the adequate acquisition time. The
results of the experimental investigation regarding the measurement methodology, drag coefficient, base pressure distribution, position of the center of pressure, and wake measurements
are presented highlighting the main natural flow features. Moreover, the pressure fluctuations measured by microphone sensors allowed to evidence the unsteadiness and possible flow configurations involving the entire rear part
An Experimental-Numerical Investigation of the Wake Structure of a Hovering Rotor by PIV Combined with a Î2 Vortex Detection Criterion
Get PDFThe rotor wake aerodynamic characterization is a fundamental aspect for the development
and optimization of future rotary-wing aircraft. The paper is aimed at experimentally and
numerically characterizing the blade tip vortices of a small-scale four-bladed isolated rotor in hover
conditions. The investigation of the vortex decay process during the downstream convection of
the wake is addressed. Two-component PIV measurements were carried out below the rotor disk
down to a distance of one rotor radius. The numerical simulations were aimed at assessing the
modelling capabilities and the accuracy of a free-wake Boundary Element Methodology (BEM). The
experimental and numerical results were investigated by the G2 criterion to detect the vortex location.
The rotor wake mean velocity field and the instantaneous vortex characteristics were investigated.
The experimental/numerical comparisons show a reasonable agreement in the estimation of the
mean velocity inside the rotor wake, whereas the BEM predictions underestimate the diffusion effects.
The numerical simulations provide a clear picture of the filament vortex trajectory interested in
complex interactions starting at about a distance of z/R = 0.5. The time evolution of the tip vortices
was investigated in terms of net circulation and swirl velocity. The PIV tip vortex characteristics
show a linear mild decay up to the region interested by vortex pairing and coalescence, where a
sudden decrease, characterised by a large data scattering, occurs. The numerical modelling predicts
a hyperbolic decay of the swirl velocity down to z/R = 0.4 followed by an almost constant decay.
Instead, the calculated net circulation shows a gradual decrease throughout the whole wake development.
The comparisons show discrepancies in the region immediately downstream the rotor disk but
significant similarities beyond z/R = 0.5
Wake control of a 3D bluff body
Get PDFBluff bodies are characterized by massive flow separation responsible of most of the pressure drag.
Three main contributions roughly determine the total amount of the body drag : 30% is due to the rolling tire, 45% derive from the rear base and 25% comes from the underbody flow and interferences. In the case of large and medium size trucks employed for short and long distance of transportations the fuel consumption can be considerable reduced through the control of the flow separation from the base. The afterbody geometry plays a key role for the drag contributions as showed by Ahmed [1]. Aider et al. [2] used passive vortex generators while piezoelectric vortex generators were investigated by Orazi et al. [3]. Continuous blowing slots as done by RoumĂ©as et al. [4] have been also used on a semi-infinite body. The present investigation considers a simplified 3D car with square-back rear shape. The wake control consist in the injection of continuous jets through four rectangular slots mounted around the perimeter of the rear part. Furthermore, four curved slots are disposed near the wheel to control the flow separation in this region. The flow analysis is performed by means of CFD commercial code (STAR CCM+Âź by Cd-Adapco). Results of this preliminary investigation will be presented. The effects of the independent rear slots orientation (ïȘi) and of the jet velocity (Vi) are analyzed as well as the effects of the jets around the wheels. The study will be also carry out on a physical model that will be tested in a wind tunnel. In figure 1 the expanded model and a portion of the longitudinal section of the rear part of the body are shown
An assessment of vortex detection criteria for 2C-2D PIV data
Get PDFThe aim of the article is to propose a robust and reliable engineering method for identifying and characterizing vortical structures within a flow field measured with a classic twocomponent PIV measurement system. Some of the most popular vortex-detection criteria are briefly presented for comparison purposes. Many of these fail if spurious vectors are present within the flow field due to poor PIV image quality. The proposed method was tested both on synthetic images of ideal vortices, having different spatial resolutions and different noise levels in order to perform a parametric assessment, and on real PIV images of a four-bladed rotor wake
Aeroelastic-structural coupling in antenna prototype for windy open-space
Get PDFThe interaction between wind and an antenna prototype for the low-frequency radio telescope of the Square Kilometer Array (SKA) is experimentally tested in the wind tunnel of the Politecnico di Torino. The tests aim to predict the antenna behaviour during working conditions, i.e. mounted by means of five contact points to a metal grid on sandy ground in the Australian desert. The wind tunnel is characterised by a circular test section having a diameter equal to 3 m and a length equal to 5 m. The height and the distance between the lateral legs of the antenna are equal respectively to 2.2 m and 1.5 m. The tests were performed at increasing wind speed up to 110 km/h. The system under analysis is an aluminium antenna composed by four parts arranged in axial symmetry and each one made of fifteen rods and small plates/wire elements. A numerical parametric model of the system is developed to numerically study the dynamic behaviour of the antenna in the frequency range of interest. The model is able to handle very high modal density and closed spaced modes in multiplicity of four because of the symmetric structure as well as the different shapes of the elements forming the antenna. The wind tunnel results emphasise the fluid-structure coupling of aerodynamics modes and the critical aspects of the boundary conditions for a good prediction of the oscillations amplitudes
Separation delay on thick airfoil using multiple synthetic jets
Get PDFHigh momentum 2D synthetic jet actuators are tested experimentally focusing the investigation on the separation delay over the airfoil NACA 0024 at high angles of attack. Four slots are present on the suction side of the airfoil. The number of slots in the downstream direction, the position and the width of the active slots and the forcing frequency were varied to investigate on their effects produced. Pressure measurements and wake analysis were performed at Reynolds number equal to 106. A weak influence was observed at low and medium incidences on the lift and drag curves. Considerable increments of the lift coefficient also associated with drag reductions were obtained at high angles of attack according to the values of the forcing frequency and the slots configuration. The influence of the slot width does not highlight great differences on the lift and drag curves. The upstream location of the slots is more effective to delay separation compared with further downstream positioning. Two consecutive active slots were effective as a single one at the same frequency while the estimated comparison at constant momentum could allow better performances. The tests were carried out operating with forcing frequencies near the resonance conditions of the system synthetic jet- cavity-pneumatic line
Experimental testing of a horizontal-axis wind turbine to assess its performance
Get PDFThis paper describes a test procedure to investigate the performance of a micro wind turbine with horizontal-axis. A 3D model of a rotor with five blades has been designed by a MATLAB software; its airfoil is optimized to efficiently work at low wind speed. The rotor is coupled to an electric generator and this equipment is tested in a wind tunnel. An anemometer is used to measure and set the desired wind speeds. Electric quantities, i.e., voltage, current and power, are acquired by a digital multimeter. A variable resistance is used to change the operating point of the generator. Preliminary results are reported that refer to the application of the proposed test procedures to a wind turbine with a 0.2 m2 swept area
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