| Summary: | The boundary layer is the main source of frictional resistance in gap flow, and the study of the flow structure characteristics of the gap flow boundary layer is of great significance for the study of gap flow theory. In this study, the PIV technique was utilized to experimentally investigate the gap flow boundary layers with Reynolds numbers of 16,587–56,870 and gap ratios of 0.6–0.8. The characteristics of the wall friction velocity, the boundary layer thickness, and the wall function of the gap flow boundary layer were analyzed, and the influences of the mean velocity of the gap flow and the gap ratio on the flow structure characteristics of the boundary layer were explored. The results show that using PIV to measure the velocity profile in the viscous sub-layer to solve for the wall friction velocity had good precision. The boundary layer thickness was inversely proportional to the mean velocity of the gap flow and the gap ratio. The wall functions of the boundary layer were as follows: in the viscous sub-layer (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mi>y</mi></mrow><mrow><mo>+</mo></mrow></msup><mo><</mo></mrow></semantics></math></inline-formula> 5.5), <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mi>u</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>=</mo><msup><mrow><mi>y</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow></semantics></math></inline-formula>; in the transition layer (5.5 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo><</mo><msup><mrow><mi>y</mi></mrow><mrow><mo>+</mo></mrow></msup><mo><</mo></mrow></semantics></math></inline-formula> 26), <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mi>u</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>=</mo><mfrac><mrow><mn>1</mn></mrow><mrow><mn>0.071</mn></mrow></mfrac><mi>t</mi><mi>a</mi><mi>n</mi><mi>h</mi><mfenced separators="|"><mrow><mn>0.071</mn><msup><mrow><mi>y</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow></mfenced></mrow></semantics></math></inline-formula>; and in the logarithmic layer (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mi>y</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>></mo></mrow></semantics></math></inline-formula> 26), <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mi>u</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>=</mo><mn>2.78</mn><mi>l</mi><mi>n</mi><mfenced separators="|"><mrow><msup><mrow><mi>y</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow></mfenced><mo>+</mo><mn>3.8</mn></mrow></semantics></math></inline-formula>. The thickness of the logarithmic layer was proportional to the mean velocity of the gap flow and inversely proportional to the gap ratio. The inner region of the boundary layer extended to <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>y</mi><mo><</mo></mrow></semantics></math></inline-formula> 0.18<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>δ</mi></mrow></semantics></math></inline-formula> or <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>y</mi><mo><</mo></mrow></semantics></math></inline-formula> 0.13(<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>h</mi></mrow></semantics></math></inline-formula>/2).
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