Separation control with elliptical air-jet vortex generators
Deepak Prem Ramaswamy, Anne-Marie Schreyer
Abstract
Abstract The flow organisation of air-jet vortex generators (AJVGs) of elliptical cross sections and their control effectiveness on a $$24^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mn>24</mml:mn><mml:mo>∘</mml:mo></mml:msup></mml:math> -compression-ramp-induced shock-wave/boundary-layer interaction was analysed on the basis of experiments at $$M_{\infty } = 2.52$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>M</mml:mi><mml:mi>∞</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mn>2.52</mml:mn></mml:mrow></mml:math> and $$Re_{\theta _c} = 8225$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>R</mml:mi><mml:msub><mml:mi>e</mml:mi><mml:msub><mml:mi>θ</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:msub><mml:mo>=</mml:mo><mml:mn>8225</mml:mn></mml:mrow></mml:math> . We investigated a circular orifice and two elliptical orifices of aspect ratios 0.5 and 2; all characterised by the same hydraulic diameter. Measurements of separation lengths from oil-flow visualisation and PIV reveal that elliptical AJVGs achieve a $$25\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>25</mml:mn><mml:mo>%</mml:mo></mml:mrow></mml:math> reduction in total separation length, which constitutes a strong improvement over the $$17\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>17</mml:mn><mml:mo>%</mml:mo></mml:mrow></mml:math> reduction achieved with the commonly used circular AJVGs. The jet-induced structures from elliptical AJVGs penetrate on average $$25\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>25</mml:mn><mml:mo>%</mml:mo></mml:mrow></mml:math> farther into the boundary layer. However, the lateral spread is limited to a maximum value equal to the inter-jet spacing in the control array, which highlights the onset of jet/jet interactions between adjacent jets in the array. A consequence of these interactions is better flow entrainment for the elliptical cases, as observed in the mean boundary-layer velocity profiles and an improved turbulent mixing (indicated by an increase in Reynolds-shear-stress magnitude).