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<p>The current experimental investigation focuses on unsteady laminar boundary layer separation over a tangent hyperbolic geometry. The geometry resembles rounded backward facing step with strong curvature. Accordingly, it is capable of producing a short separation bubble. Experiments are conducted in a special type of water tunnel suitable for relatively short stroke length. The tunnel consists of a rectangular channel with the geometry fixed at the bottom wall, a sliding piston, a diffuser and a tank. The piston is actuated through a servo drive and imparts trapezoidal velocity variation (constant acceleration from rest, constant velocity and constant deceleration to rest) onto the freestream in the channel. Effect of acceleration and velocity on bubble characteristics are studied at low Reynolds numbers (Re). Formation of primary vortex begins at the maximum pressure gradient point with subsequent development of secondary and tertiary eddies. At low Re, tertiary eddies are absent, the bubble grows slowly but the overall size attained is more for a given piston travel while the converse applies at high Re. Normalized vortex formation time scales inversely with Re. Attributes of unsteady separation like ejection of fluid particles along the line of zero vorticity and appearance of saddle point are clearly seen in the experiments. With slow start of the piston, reverse flow commences in the Acceleration Phase (AP) itself. Normalized vortex formation time in the AP collapses into a narrow band with a mean value of 1.91 when plotted against acceleration Reynolds number (<InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="707_2025_4409_Article_IEq1.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="28" /> </InlineMediaObject> <EquationSource Format="TEX">\(\textrm{Re}_a\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mtext>Re</mtext> <mi>a</mi> </msub> </math></EquationSource> </InlineEquation>).</p>

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A short laminar separation bubble over a rounded backward facing step

  • 期刊论文

  • 发表日期：

    2025年6月29日

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  • N. Ramalingam,
  • S. P. Das

摘要

The current experimental investigation focuses on unsteady laminar boundary layer separation over a tangent hyperbolic geometry. The geometry resembles rounded backward facing step with strong curvature. Accordingly, it is capable of producing a short separation bubble. Experiments are conducted in a special type of water tunnel suitable for relatively short stroke length. The tunnel consists of a rectangular channel with the geometry fixed at the bottom wall, a sliding piston, a diffuser and a tank. The piston is actuated through a servo drive and imparts trapezoidal velocity variation (constant acceleration from rest, constant velocity and constant deceleration to rest) onto the freestream in the channel. Effect of acceleration and velocity on bubble characteristics are studied at low Reynolds numbers (Re). Formation of primary vortex begins at the maximum pressure gradient point with subsequent development of secondary and tertiary eddies. At low Re, tertiary eddies are absent, the bubble grows slowly but the overall size attained is more for a given piston travel while the converse applies at high Re. Normalized vortex formation time scales inversely with Re. Attributes of unsteady separation like ejection of fluid particles along the line of zero vorticity and appearance of saddle point are clearly seen in the experiments. With slow start of the piston, reverse flow commences in the Acceleration Phase (AP) itself. Normalized vortex formation time in the AP collapses into a narrow band with a mean value of 1.91 when plotted against acceleration Reynolds number ( \(\textrm{Re}_a\) Re a ).