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Studies demonstrate that the reduced frequency <i>k</i> is influenced by the incoming wind speed <i>U</i>₀ and the rotor speed <i>n</i>. As a dimensionless parameter, <i>k</i> characterizes the stability of the flow field, which is a critical factor affecting the performance of vertical-axis wind turbines (VAWTs). This paper investigates the impact of <i>k</i> on the performance of straight-blade vertical-axis wind turbines (H-VAWT). The findings indicate that 0.05 is the critical value of <i>k</i>. The same <i>k</i> results in a similar flow field structure, yet the performance changes vary with different <i>U</i>₀. A decrease in <i>n</i> or an increase in <i>U</i>₀ leads to an increase in the average value and fluctuation of <i>k</i>, which subsequently reduces the rotor rotation torque <i>C_m</i> and decreases the maximum wind energy utilization rate <i>Cp_max</i>. This reduction in <i>Cp_max</i> weakens the stability of the flow field. Additionally, the high-speed area of the blade’s trailing edge velocity trajectory at <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>θ</mi><mo>=</mo><mn>0</mn><mo>°</mo></mrow></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>θ</mi><mo>=</mo><mn>120</mn><mo>°</mo></mrow></semantics></math></inline-formula>, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>θ</mi><mo>=</mo><mn>240</mn><mo>°</mo></mrow></semantics></math></inline-formula> expands with increasing range. Velocity dissipation in the high-speed area of the trailing edge affects the stability of the flow field within the rotor.